A drought hits the habitat of a semi aquatic bird population. All ponds dry up and fish populations decline.There are two groups of birds in the population that differ in leg lenght and diet. Long legged birds ear fish,while short legged birds eat insects. The drought has little effect on insects populations.

The main selective pressure in this scenario is the availability of food, specifically the decline in fish populations caused by the drought. This reduction in fish availability is likely to have a significant impact on the long-legged birds that primarily rely on fish as their main food source. This could lead to a higher mortality rate among the long-legged birds, putting selective pressure on the population to adapt and possibly shift their diet to insects or find alternative sources of food.